1. Field of the Invention
The present invention relates to a curable metallic paste composition suitable for the formation of three dimensional sintered bodies by rapid prototyping and manufacturing procedures. The procedures are additive in nature and create complex freeform solid objects directly from a computer model without part-specific tooling. Collectively the procedures can be termed solid freeform fabrication. The present invention also relates to a solid freeform fabrication procedure for obtaining these three-dimensional sintered bodies using this paste composition as well as the sintered body prepared by that procedure. In particular, this invention relates to a curable metallic paste composition and its use in the formation of three dimensional sintered bodies by solid freeform fabrication procedures, said paste composition containing a homogeneous mixture of (a) at least one binding agent comprised of at least one polymerizable resin, with a viscosity of less than 4000 mPa.s, measured at 25xc2x0 C.; (b) at least one initiator; and (c) at least one specific type of metal powders. The paste may be either photocurable or thermally curable.
2. Description of the Relevant Art
The creation of three-dimensional parts with complex shapes in very competitive timeframes by rapid prototyping and manufacturing procedures is known in prior art. These procedures include using stereolithography machines that employ a photosensitive liquid material which may be cross-linked or polymerized by illumination from ultraviolet (UV) laser scanning for example, so-called laser sintering machines that employ a powder raw material whereby the powder may be locally bonded by a thermal effect from infrared (IR) laser scanning for example, or machines using heating filaments or cutting out sheets,.
Another process commercialized as the Keltool(copyright) process by the assignee of the present invention produces green parts from a stereolithography master, a mold, casting and infiltration. This process is disclosed in U.S. Pat. No. 3,929,476. This process, however, requires multiple steps and is time consuming to produce a near dense part. In addition to liquids, powders, filaments or sheets, there is another range of particularly interesting materials for rapid prototyping and manufacturing applications: highly viscous materials which are not deformed by the action of gravity without necessarily being solids, hereinafter referred to as pastes. These pastes are obtained by blending a solid charge in the form of a powder, for example, a mineral, metallic or ceramic powder, into a binding agent comprised of a photosensitive or heat-cured liquid resin, such as an acrylic or epoxy photopolymerizable resin traditionally used in stereolithography. The term paste covers, in particular, materials with a very high viscosity, greater than 10,000 mPa.s or the so-called xe2x80x9cmarked thresholdxe2x80x9d materials. A xe2x80x9cthresholdxe2x80x9d material does not flow (zero gradient) as long as the shear limitation applied to it does not exceed a minimum value. A xe2x80x9cmarked thresholdxe2x80x9d is considered to be reached when the value of this shear limitation is greater than 20 Newtons per square meter.
For the formation of three-dimensional parts using these pastes, a layering or additive process is employed. The paste is spread in thin layers, with each layer being selectively solidified by a device emitting radiation, for example, a UV or IR laser, combined with galvanometric mirrors, as in stereolithography or laser sintering, respectively. Such pastes may be used for the manufacturing of metallic products by performing an additional thermal treatment after the above-mentioned formation stage. This treatment, comparable to that of parts obtained by a metal injection molding (MIM) type process, consists on one hand in eliminating the organic portion of the formed part, that is the polymer part and the potential thermodegradable additives, hereinafter referred to as xe2x80x9cdebinding,xe2x80x9d then in densifying the debinded part by sintering in order to obtain the desired mechanical properties.
However, current pastes do not allow for obtaining metallic products which present satisfactory properties. In fact, problems of cracking, swelling, bubbles or distortion appear during thermal treatment of parts formed from paste compositions and shrinkage phenomena during sintering have yet to be mastered.
In addition, it is known through U.S. Pat. No. 5,496,682 how to create sintered three-dimensional parts from a liquid composition presenting a viscosity preferably lower than 3000 mPa.s and not from a paste like the one described above, using a procedure similar to that described above, but adapted to an initially liquid composition. This patent relates essentially to the creation of parts sintered in ceramic as shown in the two examples presented. In order to maintain a good ability to flow, which is necessary for the described procedure, while maintaining a sufficient powder content, the initial liquid composition includes a photopolymerizable resin with a low viscosity, on the order of 70 mPa.s for the resin used in Example 1 and on the order of 5 mPa.s for the resin used in Example 2, and must necessarily include a dispersant.
Therefore, one aspect of the present invention is directed to a paste composition, including a binding agent charged with metallic powder, to be used in a solid freeform fabrication procedure, comprising:
a) a solidifiable binding agent comprised of at least one polymerizable resin, with a viscosity of less than 4000 mPa.s, measured at 25xc2x0 C.;
b) at least one initiator, in a concentration greater than about 0.1% by mass with respect to the mass of the resin; and
c) a mixture of at least two metallic powders, said mixture having a volumetric concentration greater than about 40% with respect to the composition, wherein said mixture of metal powders is either i) a bimodal or trimodal mixture in nature, or ii) is a majority of stainless steel with an amount of nickel boron (NiB) or nickel phosphate (NiP) compositions and combinations thereof effective to lower the sintering temperature.
Another aspect of the present invention is directed to a process for obtaining metallic products from the above-noted paste composition in comprising the steps:
a) forming a composite product comprised of thin superimposed layers of the above-noted paste, obtained by the repetition of a cycle including the following operations:
transforming the layer which was just created by polymerizing the binding agent, and
coating the transformed layer by a new non-transformed layer,
b) removing the polymer contained in the composite product obtained in the preceding step a) by a thermal treatment called the debinding step, and
c) sintering the debinded metallic product obtained in the preceding step b).
Still another aspect of the present invention is directed to a debinded and sintered metallic product obtained by the above procedure.
The metallic products obtained from the present invention permit a green part to be obtained without the time consuming steps of producing a master pattern, using a mold such as a curable silicone rubber in a room temperature vulcanizing (RTV) process, and casting. The metallic parts produced from the present invention can be sintered and infiltrated to near full density.